In construction doors are installed in structures to restrict entry, separate offices, create fire barriers, separate heating/cooling zones, form sound barriers, protect occupied areas from the elements, form privacy barriers and for other reasons. These doors are usually made from wood or metal or a combination of the two and can be mounted on a track as are pocket doors or bi-fold doors or they can be mounted on pivot hinges fastened to the floor and head of a jamb allowing the door to swing freely in either direction (double acting) but by far the most common method of mounting doors in openings is by installing two or more simple mortise hinges made up of two halves or "leaves" joined in the center by a hinge pin.
Of the many tasks faced by contractors and carpenters, the installation of doors is one of the most complex requiring a high degree of skill. Specialized tools have been developed for door installers for efficiently fitting doors in openings including routers and hinge mortise jigs, door bucks, door lock mortising machines, portable planer/jointers, and elongated levels to name a few.
Doors can be purchased in a wide variety of shapes and sizes and an endless selection of styles but for the installer they generally fall into one of two categories: either they are "pre-hung" or they are not. A pre-hung door is purchased with the hinges already mortised in the jamb and door and represents a less complicated challenge for the installer since all that is required is to mount the jamb in the opening, plumb and level and the pre-fit door will operate freely. If the door is not pre-hung, the jamb and door are separate and the installers task is more complex since he now has to mount the jambs, mortise the door and the jamb to accept their respective parts of the hinge and then fit the door to the opening. All things being equal, pre-hung doors are more expensive to buy and less expensive to install where the reverse is true for doors that are not pre-hung. Whether the door is pre-hung or not, the most efficient method of mounting it in an opening is to remove the hinge pins and separate the door from the jamb, install the respective parts of the hinge on the door and on the jambs and then position the door in the opening and insert the hinge pins.
Once the jambs and the door have been fitted with their respective parts of the hinge the installer can now lift the door into the opening, mesh the two leaves of the hinge and insert the hinge pins thereby hanging the door. If it is a pre-hung door and the jambs were properly installed there should be little or no planing or sanding to achieve a properly fitted door in the opening. If it is not a pre-hung door, the installer may have to scribe the door to the jamb, remove it, plane and/or sand the door to the scribe mark, and then re-hang it. This procedure may have to be repeated several times before an acceptable fit is achieved.
Most contractors and carpenters who specialize in door installation usually work alone. They normally make a significant investment in tools and are reluctant to allows others to use them. Although there are some tasks that can be performed more efficiently by more than one worker, the occurrence of these tasks is only occasional, and the complexities of hiring, preparing payrolls, filing taxes, buying insurance, etc., for an employee usually outweigh any benefit.
Although most craftsmen would have great difficulty defining the the laws of physics they encounter while performing their trades, they are nevertheless adept at employing short cuts and techniques for managing heavy objects such as doors when working alone. By leaving one end of the door on the floor, the craftsman only has to lift half its weight. By "cornering" a door or balancing it on one of its corners, it can easily be turned completely around. By "walking" a door or alternately balancing it on the corners of one end while advancing the airborne end forward, the object can be moved short distances with minimal effort. However, once the jambs have been plumbed and leveled and the hinges positioned, the installer must lift the entire weight of the door to complete the installation.
The height the installer must lift the door is usually less than two inches. Doors are made to fit close to the floor and only need clearance for rugs, weather stripping, thresholds, sills, etc however, exterior doors, fire rated doors, and custom doors of infinite variety can weight well over 100 pounds and the installer must maneuver the door with enough precision to lift the door to the desired height, interlock the two parts of the hinge and hold it steady while using one hand to insert the hinge pins. It is at this critical point, while lifting and holding this large object upright, that damage to the door, jamb, floor or any furniture and fixtures in close proximity, can occur from dropping the door. As previously stated, in the case of a door that is not pre-hung, the installer may have to repeat the process of placing the door on its hinges several times before an acceptable fit is achieved. Because of the bulk of some doors, two and even three men may be required to lift the door, engage the the leaves of the hinge, and insert the hinge pins. When the door is mounted for a fit and must be planed or sanded or cut for a proper fit, the same effort is required for dismounting the door, that is, lifting the door to remove the weight from binding the pins, removing the pins, and disengaging the leaves of the hinges before removing the door to a work station.
Since there is no tool currently in use to assist the craftsman when performing this task, each individual uses different aids to achieve the desired results. These include placing simple pry bars, shims, or the installers foot or fingers under the door to lift the door to the desired height and/or to take up some of the weight of the door while the pins are being set. As noted before this can lead to injury to the installer such as bruised fingers and toes, muscle strain, hernia and rupture to name a few. In the case where a replacement door is being installed above a finished floor, crude tools can mar, scratch, tear, rip or dent a finished floor surface. In other cases one or more assistants may be required to mount the door.